Diamond has many excellent properties, for example, very high hardness, chemical stability, high heat conductivity, high sound wave propagation speed, etc. At the present time, in the market, there have widely and practically been used, as polycrystalline diamond, (1) a polycrystalline diamond sintered compact comprising at least 70 volume % of diamond grains bonded with each other, (2) a diamond-coated hard material comprising a hard material the surface of which is coated with diamond polycrystal and (3) a hard material brazed with diamond polycrystal, for example,
1 cutting tools such as throwaway inserts, drills, microdrills, endmills, routers, etc., which are used for cutting working light metals such as Al, Al--Si alloys, etc., plastics, rubbers, graphite and the like;
2 rock mining tools;
3 various wear resistant tools, wear resistant jigs and environment resistant tools such as bonding tools, printer heads, dies, guide rollers for hot working, rolls for making pipes and the like;
4 various machine parts such as radiating plates;
5 various vibration plates such as speakers;
6 various electronic parts; and
7 various grinding or polishing wheels such as electrodeposited grinding wheels and dressers.
The polycrystalline diamond compact obtained by sintering diamond fine powder under ultra-high pressure has been disclosed in, for example, Japanese Patent Publication No. 12126/1977. According to a production process described in this publication, diamond powder is arranged to be in contact with a formed or sintered body of cemented carbide and sintered at a temperature of higher than the liquidus temperature of the cemented carbide under an ultra-high pressure, during which a part of Co in the cemented carbide intrudes into the diamond powder and functions as a binder metal. The thus obtained diamond compact is worked in a desired shape, brazed to various alloys and widely used for, for example, cutting tools, wear resistant tools, digging tools, dressers, wire-drawing dies, etc.
The diamond-coated hard material comprising a hard material the surface of which is coated with polycrystalline diamond has widely been used in the similar manner to the above described diamond compact. As the prior art, there are a number of publications such as Japanese Patent Laid-Open Publication Nos. 57802/1987, 57804/1987, 166904/1987, 14869/1988 and 140084/1988, in which the surface of a hard material with a suitable shape is coated with polycrystalline diamond synthesized from gaseous phase to markedly improve the wear resistance of the substrate. The diamond-coated hard material obtained by this method has a high degree of freedom in shape and a large advanatge such that it can economically be produced in a large amount, and has widely been used as, for example, cutting tools, wear resistant tools, digging tools, dressers, wire-drawing dies, etc.
Furthermore, a diamond coated layer is formed on a surface of a substrate from gaseous phase and the substrate is removed by etching to prepare a plate of polycrystalline diamond, which is worked in a desired shape and brazed to various base metals. The resulting article has been applied to, in addition to the above described uses, various vibrating plates including those of speakers, filters, window materials, etc.
At the present time, there are methods of coating the surface of a substrate with polycrystalline diamond from gaseous phase, for example, microwave plasma CVD method, RF-plasma CVD method, EA-CVD method, induction field microwave plasma CVD method, RF hot plasma CVD method, DC plasma CVD method, DC plasma jet method, filament hot CVD method, combustion method and like. These methods are useful for the production of diamond-coated hard materials.
Of the above described prior art techiques, the various tools obtained by brazing the diamond sintered compact to base metals are restricted in shape. Specifically, it is difficult in the techniques at the present time to braze the diamond sintered compact to all edge parts of, for example, a four-edge end mill with a higher precision. Thus, a round bar of diamond compact must be prepared and subjected to discharge working to obtain a desired shape, so other parts than those really needing a wear resistance are also formed of the diamond compact, resulting in a higher production cost and a lower productivity. This can similarly be said in the case of brazing a polycrystalline diamond plate.
In order to overcome the above described disadvantages, development of a diamod-coated hard material comprising a substrate worked in a desired shape, provided with, on the surface thereof, a diamond-coated layer has widely been carried out. For the diamond-coated hard material, it is first considered to use WC-based cemented carbides excellent in various physical proeprties as a substrate, and when using the WC-based cemented carbides as a substrate, it can sufficiently be expected to provide an article having a higher degree of freedom in shape and higher strength than the diamond compacts and polycrystalline diamond plate-brazed articles in a large amount and in an economical manner. Accordingly, many researchers have made efforts to improve the properties of the diamond-coated hard material, but at the present time, many of the diamond-coated tools are lacking in bonding strength of the diamond-coated layer to a substrate and the diamond-coated layer is stripped to shorten the service life, i.e. not to obtain an equal life to that of the diamond-coated hard material, in many cases. The reason therefor is given below:
1) The thermal expansion coefficients of diamond and a substrate are so different that a residual stress is caused in a diamond-coated layer and the diamond-coated layer tends to be stripped,
2) Diamond having no intermediate phase with all materials shows a low wetting property with other materials and
3) When a substrate contains a metallic element such as Fe, Co, Ni, etc., through which carbon can easily be diffused, like NC-based cemented carbides or cermets, graphite as an allotrope of diamond tends to be preferentially formed on these metallic elements and accordingly, the initial diamond nuclei generating density, during coating diamond, is lowered and the bonding strength between a diamond-coated layer and substrate is lowered, while the wear resistance of the coated layer itself is degraded.
For the purpose of solving the reason (1), there is proposed a method comprising selecting, as a substrate material, a material having the same coefficient of thermal expansion as diamond, for example, a sintered compact consisting predominantly of Si.sub.3 N.sub.4 or a sintered compact consisting predominantly of SiC, as disclosed in Japanese Patent Laid-Open Publication Nos. 59086/1985 and 291493/1986. Furthermore, it has been proposed to deposit hexagonal pillar or columnar crystals of silicon nitride on the surface of a substrate consisting predominantly of silicon nitride (Si.sub.3 N.sub.4) to form a roughened state on the surface, providing the roughened surface with a diamond coated layer, and the diamond-coated layer and substrate are rendered geometrically entangled, thus increasing the bonding strength of the diamond-coated layer, as described in Japanese Patent Application No. 269214/1990. According to these proposed methods, the bonding strength between a substrate and diamond-coated layer is markedly increased.
However, in the case of applying the resulting article to, for example, cutting tools and using them under severe conditions, breakage takes place from the substrate because the substrate of Si.sub.3 N.sub.4 or SiC is lacking in strength and the cutting tools can no longer be used.
As a countermeasure for the reason (2), the surface of a substrate is coated with an intermediate layer and further coated with a diamond-coated layer as described in Japanese Patent Publication No. 7267/1987. When a suitable material is used for the intermediate layer according to this method, the diamond-coated layer and intermediate layer are bonded with a high bonding strength. However, the inventors could not find a material for the intermediate layer, capable of obtaining a sufficient bonding strength simultaneously in the two interfaces between the substrate and intermediate layer and between the intermediate layer and diamond-coated layer, in spite of their studies to examine the bonding strength under severe conditions.
As a countermeasure for the reason (3), there has been proposed a method comprising subjecting the surface of a cemented carbide substrate to etching with an acid solution to remove metallic elements such as Fe or Co as a binder phase, as described in Japanese Patent Laid-Open Publication No. 201475/1989. In the case of carrying out the etching, however, an etched layer is sometimes present on the surbstrate surface to lower the strength of the substrate itself, and the dispersed hard phase tends to scale off or to be broken by the removal of the binder phase, thus resulting in tendency of scaling-off of the diamond-coated layer with the hard phase.
Furthermore, there has been proposed a method comprising subjecting the surface of a substrate to a scratching treatment with diamond grains or a diamond wheel and thereby improving the nuclei forming density of diamond on the surface of the substrate at the initial period of forming a diamond-coated layer, as described in Japanese Patent Laid-Open Publication No. 124573/1986.
In these proposed techniques, however, a sufficient bonding strength between a WC-based cemented carbide and a diamond-coated layer cannot be obtained and it is difficult to obtain a diamond-coated hard material having a sufficient bonding strength as a cutting tool or wear resistant tool. That is, at the present time, no one has succeeded in mass production of a diamond-coated layer having a high bonding strength to a cemented carbide substrate with a low cost.
Under this situation, the present invention aims at providing a diamond-coated hard material having an excellent bonding strength, high toughness and high degree of shaping and a process for the production of the same.